You drink half a gallon of milk a day? Wow. I'm about as white as they make 'em, and if I drank a half gallon of milk, I'd absolutely have an incident.
It was more like a dare, along the lines of: if you claim that you can't gain weight, just take whatever you are currently doing, and add a gallon of milk a day.
A jar of peanut butter a day would also work. Or just increasing your intake of calories overall.